Muslims and Christians believe there is only one God / Allah. The basic testimony of Islam is called the 'shahada', the first clause of which states that la ilaha illa Allah -- There is god but God. This is certainly a statement that Christians would affirm. But how Christians and Muslims conceptualize God in their respective theologies is actually quite different. The emphasis in the Islamic theology of God can be summarized by one word: 'tawhid', which means absolute unity. Muslims insist that there is distinction within the Godhead. God is sublimely one. Thus the Islamic polemic against Christianity has centered on the doctrine of Trinity. This is the central doctrine that causes problems for Muslims when they consider Christianity. Muslims have caricatured Christians as tritheists guilty of shirk, that is, attributing an associate to God. By believing in the Trinity, Muslims say, Christians believe in three gods. This attitude is expressed in the Qur'an: Say t trinity, Desist. It will be better for you. For God is One God (4:171). They do blaspheme who say: God is one of three in a Trinity, for there is God except One God (5:76). But every one who kws Christian theology well kws that the doctrine of Trinity was articulated precisely to oppose the idea of believing in three gods! Apparently the understanding of the Trinity was very inadequate among the Christians with whom the earliest Muslims interacted. Early Muslims, therefore, came to understand the Christian doctrine of the Trinity in very distorted, inadequate terms. It seems that some even believed that Christians worshipped Mary as part of the Trinity! This misunderstanding of the Trinity found expression in the Qur'an itself: And behold, God will say; O Jesus the Son of Mary! Didst thou say unto men, Worship me and my mother as gods in derogation of God? (5:119).
Carl Heinrich Becker (April 12, 1876 - February 10, 1933) was a German orientalist and politician in Prussia. 1921 and 1925-1930 he served as Minister for Culture in Prussia (independent). He is one of the founders of the study of the contemporary Middle East and an important reformer of the system of higher education in the Weimar Republic.